Royal babies mostly have safe, historical names which are passed down through the monarchy, with bookmaker William Hill reporting George as the favourite for a boy and, after a flurry of bets, Alexandra - one of the Queen's middle names - as the favourite for a girl, while Charlotte is also thought to be a contender.

Elizabeth, in tribute to the Queen, and Diana, to pay homage to William's late mother, are also expected to make an appearance if the baby is a girl, while Charles, in honour of the Prince of Wales, or Philip, for the Duke of Edinburgh, are possibilities if it is a boy.

The Cambridges are also likely to take inspiration from Kate's side of the family, perhaps honouring her father Michael or mother Carole.

Francis is a recurring name in Kate's family tree. It is both her father and her grandfather's middle name and Frances was her great-great-great grandmother's first name. It was also William's mother's middle name.

The Queen will undoubtedly be informed of the chosen names prior to their announcement, but is unlikely to wield a veto.

Royal writer Christopher Warwick said: ''The Queen is so down to earth that she's not likely to jump up and down and say this boy has got to be called Charles and George.

''This isn't going to be 'Granny, do you approve?' but more 'These are the names we've chosen - do you like them?'.

''It's much more of a personal thing these days.''

It is unlikely, however, that William and Kate would pick a name out of keeping with royal tradition, and they will be mindful of choosing a name that befits a future king or queen.

In the past, the approval of the sovereign has held great importance.

When the Duke and Duchess of York had their second daughter Princess Margaret Rose in 1930, they were planning to call her Ann Margaret, but changed their mind after learning that King George V disliked the name Ann.

Queen Victoria insisted that the name Albert be used as a middle name, if not a first, in honour of her beloved consort Prince Albert.

The Duke and Duchess of York took people by surprise in 1990 by naming their daughter Eugenie - a little-known name but one that has royal connections to Princess Victoria Eugenie of Battenberg.

The name of William and Kate's baby could come to symbolise an entire era, as in Georgian, Victorian and Elizabethan.

Royal babies' names are not usually revealed straight away and the public are often left guessing for several days as speculation mounts.

When Princess Beatrice was born in 1988, it was two weeks before her name was known.

When William arrived in 1982, his parents waited seven days before deciding upon and announcing his name. Diana was thought to have preferred names such as Sebastian and Oliver, while Charles was reportedly holding out for Albert.

Charles's own name, however, remained a mystery for an entire month and was only declared ahead of his christening in the Music Room of Buckingham Palace in December 1948.

In a more modern approach, William and Kate could buck the trend and unveil their chosen name immediately.

Royal babies tend to be given a number of middle names.

The Prince of Wales has four names - Charles Philip Arthur George - while the Duke of Cambridge also has four - William Arthur Philip Louis.

The Queen has three names - Elizabeth Alexandra Mary - while William's mother Diana, Princess of Wales, was Diana Frances.